1. Field of the Invention
The present invention relates to a process for preparing formaldehyde by oxidative dehydrogenation of methanol.
2. Description of the Background
Processes for preparing formaldehyde by oxidative dehydrogenation of methanol over a catalyst fixed bed made up of silver crystals are generally known (cf. Ullmanns Enzyklopadie der technischen Chemie, 3rd Edition, Urban und Schwarzenberg, Munich/Berlin, 1956, Volume 7, pages 660 to 663).
Advantageous effects which occur when using phosphorus compounds as promoters for the oxidation of methanol to give formaldehyde in the presence of a silver catalyst are also known from CN-A-85100530, DE-A-4022603 and JP-A-38227/83.
EP-A-0 467 169 describes the preparation of formaldehyde by oxidative dehydrogenation of methanol over a catalyst fixed bed which is made up of layers of silver crystals containing a pulverulent phosphorus-containing salt as promotor. This phosphorus-doped silver catalyst fixed bed is prepared by bringing it into contact with a phosphorus compound prior to the commencement of the formaldehyde preparation.
However, these processes are still capable of improvement because the catalyst continuously loses activity during use for the formaldehyde preparation, which is reflected in a decreasing yield. For this reason, the catalyst has to be replaced and regenerated after a relatively short time, for which purpose the formaldehyde preparation process has to be interrupted. Since the replacement of the catalyst fixed bed and the measures associated with the resumption of the formaldehyde preparation process are time-consuming and laborious, the economics of the process depend on how frequently this replacement has to be carried out.